Formwork for Concrete Structures by R.L.Peurifoy and G.D- By EasyEngineering.net

54 Chapter Four
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From Table 4-2, the reference bending stress is 1,500 lb per sq in.
No size adjustment is necessary because the design values in Table 4-2
are already adjusted for size. Adjustments for load-duration, moisture,
and flat use can be obtained as follows:
From Table 4-4, the adjustment factor for load-duration C D
= 1.25
From Table 4-5, the adjustment factor for wet condition C M
= 0.85
From Table 4-7, the adjustment for flat-use C fu
= 1.15
The adjusted bending stress can be calculated as follows:
F′ b
= (reference bending design value) × (C D
) × (C M
) × (C fu
)
= (1,500 lb per sq in.)(1.25)(.0.85)(1.15)
= 1,832 lb per sq in.
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Plywood
Plywood is used extensively for formwork for concrete, especially for
sheathing, decking, and form linings. Among the advantages are
smooth surfaces, availability in a variety of thicknesses and lengths,
and ease of handling during construction.
As presented later in this chapter, the plywood industry manufactures
a special plywood called Plyform specifically for use in forming
concrete structures. Plywood should not be confused with Plyform.
Plywood may be manufactured with interior glue or exterior glue. It
is necessary to use exterior-glue plywood for formwork. In all exteriortype
plywood manufactured under standards established by APA—The
Engineered Wood Association, waterproof glue is used to join the plies
together to form multi-ply panels. Also, the edges of the panels may be
sealed against moisture by the mill that makes the panels.
Plywood is available in 4-ft widths and 8-ft lengths with thicknesses
from ¼ in. through 1¹⁄8 in. Larger sizes are available, such as 5 ft wide
and 12 ft long. Plywood is manufactured to precise tolerances, within
¹⁄16 in. for the width and length of the panel. The panel thickness is
within ¹⁄16 in. for panels specified as ¾ in. or less in thickness, and plus
or minus 3% of the specified thickness for panels thicker than ¾ in.
Plywood is made in panels consisting of odd numbers of plies, each
placed at right angles to the adjacent ply, which accounts for the physical
properties that make it efficient in resisting bending, shear, and
deflection. Therefore, the position in which a panel is attached to the
supporting members will determine its strength. For example, for a
panel 4 ft wide and 8 ft long, the fibers of the surface plies are parallel to
the 8-ft length. Such a panel, installed with the outer fibers perpendicular
to the supporting members, such as studs or joists, is stronger than it
would be if the panel were attached with the outer fibers parallel to the
supporting members. This is indicated by the accompanying tables.
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